Conventionally, titanium oxide has been widely used in practice as a typical photocatalyst. Titanium oxide absorbs UV rays having a wavelength of about 400 nm or less, to thereby excite electrons thereof. When the resultant electrons and holes reach the surfaces of titanium oxide particles, the electrons and holes are combined with oxygen or water, thereby generating various radicals. The resultant radicals exert oxidizing effect to thereby oxidize and decompose substances adsorbed on the surfaces of the particles. Fundamentally, a titanium oxide photocatalyst functions as described above. Studies have been carried out on environmental purification utilizing such photo-functional properties of titanium oxide ultrafine particles, such as antibiosis, deodorization, antifouling, air cleaning, and water cleaning.
In order to improve the photocatalytic properties of titanium oxide particles, the following means have been proposed.
(1) Reduction of Particle Size
Reduction of particle size is very effective for preventing recombination of generated electrons and holes.
(2) Enhancement of Crystallinity
Enhancement of crystallinity is effective for increasing the rate of diffusion of generated electrons and holes toward the surfaces of particles.
(3) Separation of Electrons and Holes
Generated electrons and holes are separated from each other, to thereby increase the ratio of electrons or holes which reach the surfaces of particles.
(4) Regulation of Band Gap
When a band gap is reduced (i.e., maximum absorption wavelength is increased) through, for example, incorporation of small amounts of impurities, percent utilization of a light source containing small amounts of UV rays, such as sunlight or a fluorescent lamp, can be increased.
In order to attain the aforementioned means (4), recently, various studies have been performed on a photocatalyst which responds to visible light.
For example, as disclosed in JP-A-9-262482 (the term “JP-A” as used herein means “unexamined published Japanese Patent Application”), anatase-type titanium dioxide of high catalytic activity is modified through ion implantation by use of a metal such as chromium (Cr) or vanadium (V), so as to shift the maximum light absorption wavelength of the titanium dioxide to a longer wavelength, thereby causing the titanium dioxide catalyst to function under irradiation with visible light. However, the aforementioned ion implantation by use of a metal requires a large, expensive apparatus and is industrially impractical.
JP-A-2001-72419 discloses titanium oxide designed such that, in the case where the half-width of the peak of titanium of the titanium oxide—which is present within the bond energy of 458 to 460 eV—is measured four times by means of X-ray photoelectron spectroscopy, when the average of the half-widths of the peak of titanium at the first and second measurement is represented by “A,” and the average of the half-widths of the peak of titanium at the third and fourth measurement is represented by “B,” the index X (i.e., B/A) becomes 0.97 or less. However, since powder of the titanium oxide has low activity and is colored, application of the titanium oxide powder is limited. Therefore, the powder is not suitable for use in a coating material which requires transparency.
Conventional photocatalysts which respond to visible light are difficult to use in practice, since they require a strong light source such as a xenon lamp in order to exhibit their catalytic properties sufficiently. Therefore, it would be greatly advantageous to provide a photocatalyst which exerts sufficient effects when irradiated with light from a conventional inexpensive light source; for example, a light source used in a room, such as a day white fluorescent lamp.
International Patent Publication WO94/11092 discloses a method for treating bacteria or malodorous substances by applying a photocatalytic thin film containing a semiconductor of, for example, titanium dioxide onto the inner wall of a sickroom or a residential space. However, the publication does not disclose a method for imparting activity to titanium dioxide and the photocatalytic activity of particles of the titanium dioxide. Provided that typical titanium dioxide is employed, when the photocatalytic thin film is irradiated with light from a light source containing small amounts of UV rays, such as a fluorescent lamp, the activity of the thin film is expected to be lower than that of the aforementioned visible-light-response-type photocatalyst.
Attempts to utilize photocatalytic properties of titanium oxide fine particles have led to proposals of a method of kneading titanium oxide fine particles into an easily handled medium such as a fibrous material or a plastic formed article, and a method of applying the fine particles to the surface of a substrate such as cloth or paper. However, not only harmful organic substances or environmental contaminants, but also media such as fiber, plastic, and paper are easily decomposed and impaired by the strong photocatalytic action of titanium oxide, and, in view of durability, this stands as an obstacle to practical use. Also, since titanium oxide fine particles are easily handled, a coating material prepared by mixing titanium oxide fine particles and a binder has been developed. However, an inexpensive binder exhibiting the durability sufficiently high to overcome the problem (obstacle) of action on the media has not yet been found.
JP-A-9-225319 and JP-A-9-239277 disclose a technique of preventing and suppressing the deterioration of a resin medium or a binder that would otherwise occur because of the strong photocatalytic action of titanium oxide particles, where there is proposed, as means therefor, a method of bearing a photoinactive compound containing an element such as aluminum, silicon, or zirconium on the surfaces of titanium oxide particles such that the compound assumes a form of archipelago with steric hindrance to thereby suppress the photocatalytic action. However, this method is disadvantageous in that, although the photoinactive compound is supported so as to form an archipelago form, a portion subjected to the strong photocatalytic activity of titanium dioxide is present at a specific site of the resin medium or binder.
JP-A-10-244166 proposes photocatalytic titanium oxide prepared by coating the surface of titanium oxide with porous calcium phosphate. However, in this case, as has been pointed out, the photocatalytic performance decreases because of the coating film; i.e., the calcium phosphate layer.
International Patent Publication WO99/33566 discloses a titanium dioxide fine particle powder where a porous calcium phosphate coating layer is formed at least on a portion of the surfaces of titanium dioxide fine particles, and an anionic surfactant is present in the interface between the layer and the surface of the fine particles.
With respect to a slurry containing titanium oxide having photocatalytic activity, JP-A-10-142008 discloses an anatase-type-titanium-oxide-containing slurry obtained by subjecting a titania sol solution, a titania gel, or a titania sol·gel mixture to heat treatment and pressure treatment in a closed container, and then to dispersion using ultrasonic waves or stirring.
JP-A-11-343426 discloses a photocatalytic coating material having excellent dispersion stability, which is a photocatalytic coating material containing titanium oxide and silica sol in a solvent, where the titanium oxide has a Raman spectrum peak in the range from 146 to 150 cm−1, and anatase-type titanium oxide accounts for 95 mass % or more of the titanium oxide.
As described above, although several techniques have been disclosed, there have not yet been provided, in an industrially useful manner, photocatalytic powder and slurry which exhibit photocatalytic properties and which satisfy requirements for both durability and dispersion stability when used in combination with an organic material.